Critter and Guitari has a tutorial on how to create a Critter Board 4 voice FM synth in a cigar box.
The instrument is a cigar box synthesizer. It is made out of a Critter Board, a small audio amp, and some other components we had kicking around. Since it is a synthesizer, we use the version of the Critter Board with on-board flash (no card socket).
The project utilizes all the features that make the Critter Board unique, such as the volume control header, the sound output header (you can listen on the speaker, or plug it in to a bigger amp), and most of the I/O lines.
Source code and .hex files are available for download.
Check the tutorial for detailed instructions with pictures.